To satisfy demands for wireless data traffic having increased since commercialization of 4th-Generation (4G) communication systems, efforts have been made to develop improved 5th-Generation (5G) communication systems or pre-5G communication systems. For this reason, the 5G communication system or the pre-5G communication system is also called a beyond-4G-network communication system or a post-Long Term Evolution (LTE) system.
To achieve a high data rate, implementation of the 5G communication system in an ultra-high frequency (mmWave) band (e.g., a 60 GHz band) is under consideration. In the 5G communication system, beamforming, massive multi-input multi-output (MIMO), full dimensional MIMO (FD-MIMO), an array antenna, analog beamforming, and large-scale antenna technologies have been discussed to alleviate a propagation path loss and to increase a propagation distance in the ultra-high frequency band.
For system network improvement, in the 5G communication system, techniques such as an evolved small cell, an advanced small cell, a cloud radio access network (RAN), an ultra-dense network, a device to device (D2D) communication, a wireless backhaul, a moving network, cooperative communication, coordinated multi-points (CoMPs), and interference cancellation have been developed.
In the 5G system, advanced coding modulation (ACM) schemes including hybrid frequency-shift keying (FSK) and quadrature amplitude modulation (QAM) modulation (FQAM) and sliding window superposition coding (SWSC), and advanced access schemes including filter bank multi carrier (FBMC), non-orthogonal multiple access (NOMA), and sparse code multiple access (SCMA) have been developed.
A frequency band is roughly classified into a licensed frequency band (i.e., a permitted frequency band) and an unlicensed frequency band (i.e., a frequency band for which permission is not required), depending on whether an exclusive right for its use is granted to a particular operator. The licensed frequency band is available only by the particular operator, such that the particular operator may transmit and receive data at a desired point in time. On the other hand, the unlicensed frequency band is available to everyone, such that an operator (or device) that is to use the unlicensed frequency band needs to use the unlicensed frequency band after determining whether there is no operator (or device) currently using the unlicensed frequency band.
FIG. 1 is a flowchart of a process in which a terminal transmits data or a control signal to a base station in a communication system using a licensed frequency band.
A terminal 103 sends a scheduling request to a base station 101 in operation 111. The scheduling request means a request for a resource that is used for the terminal 103 to transmit data or a control signal to the base station 101 in an uplink (UL). The base station 101 performs scheduling in response to the scheduling request of the terminal 103 and transmits a scheduling result to the terminal 103 in operation 113. The scheduling result may be, for example, an UL grant. If the base station 101 sends the UL grant to the terminal 103, the base station 101 may allocate a point in time to transmit data, etc., a frequency resource, and so forth to the terminal 103. The terminal 103 having received the UL grant transmits the data at the point in time allocated by the base station 101 in operation 115. The base station 101 receives and decodes the data transmitted from the terminal 103. Once the decoding has been successfully performed, the base station 101 sends an acknowledge (ACK) signal to the terminal 103. However, if the decoding has failed, the base station 101 sends a negative acknowledge (NACK) signal to the terminal 103. Even if the decoding has failed, the base station 101 soft-combines the received data with subsequently re-transmitted data without discarding the received data, thereby increasing the success rate of data reception.
Such a scheme for increasing the success rate of data reception by soft-combining previously received data with re-transmitted data without discarding the received data is referred to as a hybrid automatic repeat request (HARQ) scheme. The HARQ scheme may be roughly classified into a synchronous HARQ scheme and an asynchronous HARQ scheme. In the synchronous HARQ scheme, a base station is synchronized with a terminal by fixing a point in time when the base station sends an ACK or NACK signal to the terminal and a point in time when the terminal transmits new data or re-transmits existing data to the base station. Thus, the base station or terminal using the synchronous HARQ scheme does not need to transmit information about a point in time to transmit data and a point in time to send an ACK or NACK signal.
On the other hand, in the asynchronous HARQ scheme, the terminal and the base station inform each other of points in time to transmit data before transmitting the data, and then transmit the data at the informed points in time. That is, in the asynchronous HARQ scheme, the data transmission points in time may be flexible.
FIG. 2 is a diagram showing an example of exchange of data and an ACK/NACK signal between a base station and a terminal that use the synchronous HARQ scheme.
Referring to FIG. 2, once the terminal transmits UL data in 201, 203, and 205, the base station sends an ACK or NACK signal indicating a decoding result with respect to the data to the terminal in 211, 213, and 215 after three subframes 231. The terminal then transmits new data or re-transmits previous data in 221, 223, and 225 after three subframes 233 according to the decoding result. The number of subframes, 3, is a preset value, and may be changed.
FIG. 3 is a flowchart showing transmission and reception of data or a control signal between a terminal and a base station that use the synchronous HARQ scheme in an unlicensed frequency band.
FIG. 3A is a flowchart showing a case where any device does not use the unlicensed frequency band at a point in time when the terminal is to use the unlicensed frequency band.
FIG. 3B is a flowchart showing a case where a device uses the unlicensed frequency band at a point in time when the terminal is to use the unlicensed frequency band.
A process in which the terminal 103 sends a scheduling request to the base station 101 in operation 111 and thus the base station 101 sends a UL grant to the terminal 103 in operation 113 is the same as shown in FIG. 1, and thus will not be described now. However, FIG. 3 is different from FIG. 1 in a sense that the base station 101 allocates the unlicensed frequency band to the terminal 103.
The terminal 103, because of using the synchronous HARQ scheme, checks a state of the unlicensed frequency band at a preset point in time. That is, to avoid interference or collision with another device, before sending a signal, the terminal 103 may perform listen before talk (LBT)-based clear channel assessment (CCA) which includes sensing a power level of a channel or carrier in the unlicensed frequency band to be used and determining whether the channel or carrier is available. If the channel is in a clear state, the terminal 103 may send a signal. However, if the channel is in a busy state, the terminal 103 may not send a signal. Depending on a scheme for performing LBT, a device may be classified into frame based equipment (FBE) and load based equipment (LBE). Hereinbelow, performing CCA may have the same meaning as performing LBT.
In FIG. 3A, a state of the unlicensed frequency band (i.e., a channel state of the unlicensed frequency band) is a state where any device does not use the unlicensed frequency band, that is, a clear state 301. On the other hand, in FIG. 3B, the state of the unlicensed frequency band is a state where another device uses the unlicensed frequency band, that is, a busy state 311.
When the unlicensed frequency band is in the clear state, operation 115 where the terminal 103 transmits data and operation 117 where the base station 101 sends an ACK or NACK signal are the same as operations 115 and 117 of FIG. 1.
In contrast, when the unlicensed frequency band is in the busy state, the terminal 103 may not transmit data at a preset point in time as in 313. However, the base station 101 determines that the terminal 103 surely transmits data, and thus performs decoding, but fails. The base station 101 having failed in the decoding sends a NACK signal to the terminal 103 in operation 315.
As described with reference to FIG. 3, since the unlicensed frequency band may be in the busy state at a preset point in time, a problem may occur as below. Particularly, in the synchronous HARQ scheme where points in time to transmit data, etc., are preset by synchronization, the transmission points in time may not be freely changed, making it difficult to use the synchronous HARQ scheme in the unlicensed frequency band.
For example, the terminal 103 may distinguish the NACK signal sent in operation 117 of FIG. 3A from the NACK signal sent in operation 315 of FIG. 3B. In FIG. 3A, since the terminal 103 has transmitted data, the base station 101 may soft-combine the received data with re-transmitted data without discarding the received data, thus increasing a decoding success rate. In FIG. 3B, the terminal 103 does not transmit data, such that data received by the base station 101 may be an interference or noise signal that needs to be discarded. The problem occurs because the base station 101 does not know whether the terminal 103 has transmitted data. That is, since the base station 101 does not know whether to discard data received from the terminal 103 or to store the received data in a buffer for soft-combination, the problem occurs.